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I've been cycling for several months now. However, I have had the misfortune of having two bicycles stolen from me, so I've never had a single bicycle for more than a few months. With time passing, I've been thinking I might need to pump up my tires some. The thing is, I don't experience any degradation of my cycling experience so far, and by a cursory unexperienced glance the tires don't seem deflated (I think; I can't quite say if there's any sag compared to when I bought them).

So, how do I know whether it's time to pump the tires up? Should I just do it regularly every X weeks/months?

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    You need to pump up your tires at least weekly -- more often for narrow tires, and less often for super-wide ones. Generally speaking the tire pressure should be near the top of the range listed on the tire sidewall, and no less than the bottom of the range. – Daniel R Hicks May 18 '16 at 11:58
  • Not all tires have a bottom of the range. "Max: x psi" is not uncommon. – Batman May 18 '16 at 17:51
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The correct tire pressure for you is typically not whats written on the tire sidewall. That's an arbitrary number determined by the marketing and legal departments at the tire manufacturer, not the engineers (usually it leads to an overinflated tire, which can damage the wheel and reduce control of the bike). You'll have to play with the pressure to get a properly inflated tire. Various manufacturers have rough guidelines for what pressures are good based on weight and tire size, but they're just guidelines -- you have to play with it on your own, since tire pressure is a personal preference (given that it is not under or over inflated -- a properly inflated tire absorbs hazards, doesn't pinch flat and has low rolling resistance, whereas an under inflated tire can pinch flat and also damage the wheel). A rule of thumb is that the rear tire should be 10% higher pressure than the front due to the load difference. You may also use different pressures depending on season or other conditions.

As for how often you need to check your tire pressure, note that unless the tire is severely underinflated, you won't notice it by visual inspection or by just pushing on the tire (depending on what pressures you're running). I'd suggest using a floor pump with a pressure gauge and associating proper inflation with particular pressures for a given tire.

You lose pressure from road hazards (e.g. if you hit a bump, some air might come out) as well as leaks (inner tubes and tires are porous to some extent, and valves leak a bit). The rate at which you lose air depends on your components and your rides. Ideally, you'd check before every ride, but for most people, weekly is a good enough option.

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    The sidewall max pressure shouldn't damage anything in itself unless the rims are complete junk. That's not to say it's the right pressure for a particular rider, but if you're heavy it's not a bad place to start. – Chris H May 17 '16 at 16:27
  • Not necessarily complete junk, but there are a decent number of riders riding bicycles not suitable for them. For example, if you're 300+ pounds, you shouldn't be on some low spoke count ultra light thing. And eventually, you're going to get unlucky even with good components. – Batman May 17 '16 at 17:01
  • True enough if its overloaded. At around 2/3 that weight plus luggage, getting close to the sidewall pressure means the difference between having to pump up every week and preferring to. Anyway that's a minor quibble that wasn't meant to detract from voting up. – Chris H May 17 '16 at 17:35
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    I don't see how you can (in a tube-type tire) lose pressure due to road hazards without actually having a puncture. – Daniel R Hicks May 18 '16 at 12:01
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If you use your bike everyday, i would say you need to whack some air in every couple of weeks. If you have left it sitting for a few weeks, it will need air. Most tyres have a pressure rating on the side- find yours and use it if you can. You'll be amazed at how much faster it will feel, and its also safer in terms of handling.

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I agree with Batman's answer and would add to it that weekly pressure checking may not be enough depending on your setup and riding goals.

I can get away with a weekly topping off of my tubes under 28mm conti road tires that I'll run at 100psi.

If it's snowing/raining/whatever and I decide to run them at 80psi, they will be too soft after a week.

I cannot do the same with my tubular cross bike that I run at 40psi. I need to carefully check the pressure before every ride.

Fat bikes are probably different too. I don't own one but I imagine the large volume can stay for quite awhile.

As far as knowing when, you should start feeling the tires after you fill them to pressure with an accurate gauge. By feel I mean squeeze them, hard. Really see how stiff they are. You'll build an intuitive feel for what the right pressure feels like and be able to tell when they need air, quickly, by just grasping them.

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